This invention relates to a metal sheet forming process with hydraulic counterpressure, and more particularly a process where a punch is urged on a blankpiece (called as "material" after) into a die filled with liquid therein, and hydraulic counterpressure generated thereby is utilized to serve as pressing a flange of a material or an axial direction of a side wall of a product.
A deep drawing process is for producing deep cup like products from the plate material, and limit of breakage in the deep drawing process is determined by transmission ability in the side wall of the forming die with respect to shrinkage resistance, bending resistance and friction resistance at the flange portion and the die shoulder portion. Therefore it is necessary for increasing the limit of the breakage to decrease these resistances required to the deep drawing, and increase the available transmission power in the side wall. However an ordinary metal molding process has a limit in the amount of a once drawing formation, though the material to be processed has high drawing quality, and the limit of the drawing ratio is around 2.0 to 2.3 at best.
As a way for increasing the transmission power at the side wall of the drawing device, there is a process which utilizes hydraulic counterpressure. This process, in principle, comprises directly urging the material by means of the punch into a hydraulic pressure chamber which is provided under a die and filled with the liquid, and utilizing the hydraulic pressure generated thereby to cause the material to follow the punch in shape. As actual practices, one is a process which furnishes a packing on an upper surface of the die to contact a lower surface of the material and avoids leakage of the liquid in order to maintain high the hydraulic counterpressure, and the other is a process which does not furnish the packing as said, and presses the material into the die and positively flows out the liquid from the flange so that the liquid is discharged from a releasing space between the die and a blank holder.
Depending upon such a sheet forming process with the hydraulic counterpressure, in the former process the transmission power is increased by friction-keeping-effect in the forming side wall, and especially in the latter process when the liquid is forcibly discharged from the flane, friction-reducing-effect may be provided at the lower face of the blank material. Therefore the limit of the drawing ratio is increased in comparison with the deep drawing process by an ordinary metal mold. Particularly in the latter process, since the friction-reducing-effect is active, the drawing ratio is in general higher than in the former process.
Even if the hydraulic pressure is enough for the friction-keeping-effect in the latter process and since this process pulls the flange portion into a space between the die and the punch, a diameter of the material is large so that the resistance of the flange portion is large accordingly, and then breakage occurs at the die shoulder where the friction-keeping-effect could not exist.
By the latter process, the friction-reducing-effect is only obtained between the material and the die, and fairly large friction resistance exist between the material and the blank holder. Therefore in the conventional drawing with the hydraulic counterpressure, the drawing ratio is around 2.6 to 2.9, and a higher limit of the drawing ratio could not be expected.
The drawing through once process has a certain limit, and therefore when container or vessel of the deep drawing is formed, the drawing work is divided into several steps where the plate is drawn into a product of determined depth (first drawing), and subsequently this product by the first drawing is subjected to several deep drawings (redrawing). In the redrawing process, there are a direct redrawing process and a reverse redrawing process, and a process of incorporating the first drawing and the reverse redrawing is often used, since the number of bendings is lesser by twice if the bending degree is the same. A well known one is a continuous reverse redrawing process which employs a first drawing punch serving also as a redrawing die, a blank holder of cushion type, a first drawing die and a redrawing punch.
However, since such ordinary continuous reverse redrawing process depends upon the metal mold drawing, the drawing ratio is low, that is, limitations are that the redrawing ratio is around 1.3 and the total drawing ratio is around 2.6. For improving these limitations, after the first drawing, the formed product is subjected to an intermediate annealing. Depending upon this process, a product by the redrawing is once taken out from the pressing step, and therefore an entire process is not continuous so that the processing is not efficient. Although the intermediate annealing step intervenes, the improvement is that the redrawing ratio is around 1.8 to the maximum and the total drawing ratio was around 3.5.